1. Technical Field
The present invention relates to a switching converter, and more particularly, to a switching converter using a synchronous rectifier, and an alternating current (AC) adaptor using the same.
2. Description of the Related Art
Recently, in accordance with miniaturization, lightness, and demand for improvement of performance of a laptop computer all over the world, improvement of system specifications such as building of a multimedia system, an increase in a speed of a central processing unit (CPU), an increase in a memory, and the like, has inevitably been continuously demanded.
In addition, as capacity for resources of the respective system specifications increases, even though power of 45 to 50 watts (W) has been currently used in an alternating current (AC) adaptor for the laptop computer, the demand for high capacity of 60 watts, 75 watts, and 80 watts or more, microminiaturization and slimness for portability, and high efficiency has gradually increased in the AC adaptor for the laptop computer.
Further, the reason why the efficiency of the AC adaptor should be increased is that when the efficiency is increased, internal power loss is decreased, which means that internal heat generation is low, such that the AC adaptor may be miniaturized.
However, as the most typical scheme currently used in the AC adaptor, there are a flyback circuit scheme and a resonant scheme. Here, in the case of the flyback circuit scheme, since hard switching in which crossing between a turn-off voltage Vds and a turn-on current Ids of a metal oxide semiconductor field effect transistor (MOSFET), which is a semiconductor device, is large is performed, power loss is large. Meanwhile, since the resonant scheme may decrease switching loss, it is effective for miniaturization and lightness. However, in the case of the resonant scheme, since a voltage and a current are formed in a sinusoidal wave shape, a control property is bad, and large voltage and current stresses are applied to a switching device.
Therefore, a synchronous rectifying scheme using a synchronous rectifier (SR) has been recently spotlighted due to an advantage such as high efficiency. As the synchronous rectifier, which is a rectifying apparatus obtaining a load current always flowing in a predetermined direction by vibration or a contact in synchronization with AC power, a field effect transistor (FET) having small turn-on resistance has been generally used instead of a diode in order to minimize power loss according to a turn-on operation of an output diode in a flyback circuit and increase efficiency. The FET serves to be turned on only for a period in which the diode is turned on to minimize the power loss according to the turn-on operation of the diode.
FIG. 1 is a diagram showing an example of a flyback circuit using a synchronous rectifying scheme according to the related art; and FIG. 2 is an operation waveform diagram of the flyback circuit shown in FIG. 1.
As shown in FIG. 1, the flyback circuit using a synchronous rectifying scheme according to the related art includes a transformer T inducing primary energy to secondary side, a switch SW switching a primary voltage of the transformer T, and a synchronous rectifying switch SR SW rectifying a secondary voltage of the transformer T.
The flyback circuit using a synchronous rectifying scheme according to the related art configured as described above is operated in a continuous mode (CCM) and a discontinuous mode (DCM). In the case in which the flyback circuit is operated in the continuous mode, when a gate voltage of the switch SW is controlled, a primary current Ia of the transformer T is increased in a linear function form in a period in which the switch SW is turned on. In this case, energy is accumulated in a primary coil of the transformer T in the period in which the switch SW is turned on, and a polarity of the transformer T is changed at a point in time in which the switch SW is turned off, such that an induced current Ib flows to the secondary side of the transformer T.
A voltage denoted by SR Sensing Voltage in FIG. 2 is a negative voltage by a conducting current of a diode D when the switch SW is turned off and has a waveform that is the same as an operation waveform of the switch SW. In addition, a signal denoted by SR GATE Pulse in FIG. 2 indicates an operation waveform of the synchronous rectifying switch SR SW. In the case in which the voltage (SR Sensing Voltage) drops to a reference voltage, that is, a set direct current (DC) voltage or less when it is changed from a “positive (+)” voltage to a “negative (−)” voltage, the synchronous rectifying switch SR SW is turned on, and in the case in which the voltage becomes the reference voltage or more when it is changed from the “negative (−)” voltage to the “positive (+)” voltage as a secondary current Ib decreases, the synchronous rectifying switch SR SW is turned off.
However, in this case, since the voltage (SR Sensing Voltage) is rapidly changed from the “negative (−)” voltage to the “positive (+)” voltage, a turn-off operation of the synchronous rectifying switch SR SW is delayed by a time required to detect the negative voltage, such that a period in which the switch of the primary side and the synchronous rectifying switch SR SW of the secondary side are simultaneously turned on is generated.
This becomes a factor decreasing efficiency and stability of a system. Therefore, a technology of performing switching so that the turn-on period of the switch SW of the primary side and the turn-on period of the synchronous rectifying switch SR SW of the secondary side are not overlapped with each other has been urgently demanded.